NMDA receptor antagonists exhibit excellent neuroprotective efficacy in experimental models of stroke; however, clinical trials in stroke and traumatic brain injury with NMDA antagonists have not been successful. The failure of the currently available drugs in clinical trials can be primarily attributed to their severe side effects, which necessitate the use of significantly lower than effective doses. The key issue is to develop drugs that reduce NMDA activity without these dose-limiting side effects. This can be achieved by subtype (NR1or NR2) specific antagonists that have been clearly established to possess better side effect profiles. The currently available subtype specific compounds, however, have not been effective due to a number of problems such as activity at other receptors, insolubility, and/or inefficiency in crossing in blood brain barrier. In the proposed research we will use a promising new pharmacological approach, based on the use of RNA ligands as subtype specific antagonists for the NMDA receptors, which will address the problems with the currently available drugs. The advantage of using RNA ligands is that they can be evolved from a large pool of RNA ligands with random sequences (1015 ligands) to bind with high specificity for any target protein. Additionally, these are water soluble and have been shown to cross the blood brain barrier. The selection protocol will involve an iterative process of positive selection based on the affinity of the RNA ligand for a given subunit of the NMDA receptor followed by amplification using RT-PCR. The RNA ligands thus obtained are further selected for specificity by a negative selection step wherein those with no affinity for the other subunits of NMDA receptors are selected. The RNA ligands selected by the above steps need to be characterized to identify those with antagonistic property. This process will be greatly facilitated by the fluorescence based assay that we have developed in my laboratory. This assay provides high throughput readout of the functionality of the ligand, namely agonists or antagonist. The candidate RNA antagonists identified by the assay will be tested for antiexcitotoxic effects using in vitro and in vivo models in the Co-PI's laboratory, thus providing the basis for future preclinical and clinical trials.